Reducing equipment in size using electronic devices has prompted the use of high-density electrical circuits. Since the density of heat produced by electronic equipment increases with increasing density of electronic devices in it, axial-flow blowers or oblique-flow blowers are used to cool electronic equipment.
As shown in FIG. 11, in a conventional blower, an annular wall 2 is formed away from the end of a blade of an axial-flow fan 1, which rotates about a shaft 4, thus causing an air flow 5 from the suction side to the discharge side when a motor 3 is energized, that is, the blower is in operation.
When the blower is in operation, however, the velocity of the air flow increases on the back pressure side at the blade end, so that under the influence of secondary flows between blades, a low-energy region occurs on the blade trailing edge side, where the velocity is converted to pressure energy.
In the low-energy region, energy loss is significant and air flow easily separates from blade surfaces, over which vortices occur, thus increasing turbulent flow noise. Thus the region poses a problem of an increase in noise level and a deterioration in static pressure-flow rate characteristic (hereinafter referred to as the P-Q characteristic).
The phenomenon mentioned above is frequently observed, especially when a fan exhibits stall conditions because large leakage vortices occur at the end of a blade under the action of flow resistance (system impedance) on the discharge side.
U.S. Pat. No. 5,707,205, previously obtained by the applicant of the present invention, discloses that by sucking laminar air flow inside an annular wall through a slit therein when a blower is in operation, a blower inhibits leakage vortices and rotation stall from occurring at the end of a blade to improve the P-Q characteristic and reduce noise.
PCT-based Japanese Patent Laid-Open No. 6-508319 and U.S. Pat. No. 5,292,088 disclose that a blower is arranged so that vortices of air flowing through a plurality of rings, spaced apart from each other around an axial-flow fan, increase the air flow rate.
U.S. Pat. No. 5,407,324 discloses that a blower is arranged to make it possible for air to flow inside and outside a housing by inclining to the direction of air flow the internal perimeter of a plurality of annular plates, stacked around an axial-flow fan.
However, common blowers for personal computers and workstations, which are made rectangular with standardized dimensions to reduce their costs, have external dimensions of 60 mm square to 92 mm square. Thus it is not desired that a blower be significantly changed into a round shape by, for example, making annular plates 7.sub.1 to 7.sub.5, forming the annular wall 2, circular as shown in FIG. 12.
U.S. Pat. No. 5,707,205 also discloses a blower whose annular wall 2 is shaped so that its sections corresponding to the middle of the upper, lower, right, and left sides of a rectangular casing body 15 are flush with the casing body 15 as shown in FIGS. 13a and 13b. However, only making the contour of the annular wall rectangular as shown in FIGS. 13a and 13b causes the effect of sucking laminar air flow inside the annular wall through each slit 6 to be slightly lessened, compared with an annular wall which has a round contour as shown in FIG. 12. Thus the effect of improving the P-Q characteristic and reducing noise cannot fully be provided. The casing body described by U.S. Pat. No. 5,707,205 also has a problem of low mechanical strength and the like, because the sides of the annular wall are thinner than the other sections.
Every blower mentioned above improves a fan characteristic by sucking air around a fan. The applications only describe the arrangement of rings (annular plates) around a fan, not the shape of the fan. To fully exhibit the characteristic of a fan, its shape must be devised.
A method has generally been used which predicts the performance of a fan or determines the three-dimensional shape of a fan appropriate for use conditions by cutting a fan blade through the surfaces of cylinders concentric with the axis of rotation of the fan, developing the surfaces, converting a fan blade into a plane infinite straight-line series, and applying to the series a straight-line airfoil system theory suggested for aircraft and the like.
However, a problem with the method is that the actual performance of a fan becomes lower than that predicted by calculations under the influence of leakage vortices at the ends of blades when flow resistance higher than a given level acts on the blower.
To solve this problem by modifying the shape of the end of a blade, some fans, including one disclosed in Japanese Patent Application Laid-Open No. 6-307396, are arranged so that aerodynamic performance is improved and noise is reduced by positioning the cross-sectional section at the end of an outer blade of the fan on the leading edge side and providing an upwardly curved one-sided curved section and an arcuate section following the one-sided curved section only on the pressure surface side.
Some blowers, including one disclosed in Japanese Patent Application Laid-Open No. 8-121391, are arranged so that aerodynamic noise is reduced by curving the periphery of a blade.
Some hydraulic apparatuses, including one disclosed in Japanese Patent Application Laid-Open No. 8-284884, are arranged so that by cutting the back side of the end of a moving blade a given height from the tip and forming a thin-walled section of a constant thickness on the back side, fluid leakage from a tip clearance is reduced, thus improving the efficiency of an axial-flow blower.
However, if the above-described fan shapes according to prior art, which assumes that no air flows in from outside an annular wall, are applied to an arrangement where air is sucked from outside an annular wall, no satisfactory performance is exhibited.
Although U.S. Pat. No. 5,407,324 discloses an arrangement of the rings, the arrangement is not acceptable in terms of mass productivity, strength, and accuracy.
It is an object of the present invention to provide a blower which exhibits an improved P-Q characteristic and reduces noise as a blower in FIG. 12 whose annular wall has a circular contour even when substituted for a conventional rectangular blower and which has practically necessary strength.
It is another object of the present invention to optimize the shape of a fan blade and that of an annular wall of a blower which sucks air inside the wall through slits provided therein to improve aerodynamic performance and strength and reduce cost by increasing mass productivity.